A GaAs-Schottkey gate type FET (MES-FET) is used as a high-power and high-frequency device or a low noise device in a microwave communication system. For a high power device, a MES-FET which is fabricated to have a recess structure, that is a portion surrounding a gate electrode which is recessed, has been proposed. In such a MES-FET of the recess structure type, it is known that a double recess structure having two recesses is especially useful or height output property.
A conventional MES-FET includes a semi-insulation GaAs substrate, an n-GaAs active layer, an ohmic electrode, an insulation layer and a gate electrode.
In fabrication, the n-GaAs function layer is formed on the semi-insulation substrate, and the ohmic electrode for a source/drain electrode is formed on the n-GaAs active layer. After that, the insulation layer is entirely formed on the substrate thus processed.
Then, a photoresist layer is formed on the insulation layer, and the insulation layer is etched to form a gate hole thereon by using the photoresist layer as a mask.
Then, the n-GaAs active layer is etched to form a first recess thereon by etching, and the insulation layer is etched on the side surface. After that, the n-GaAs active layer is etched to form a second recess.
Finally, gate metal is evaporated and lifted off to form the gate electrode, whereby the GaAs MES-FET is completely fabricated.
In the GaAs MES-FET fabricated by the conventional method, however, there are disadvantages in that the depth of the recesses is difficult to be stably constant as specified, because the recesses are formed by etching. Therefore, drain current tends to vary in wide range.
Further, a drain current or a break-down voltage for a gate is fluctuated, because the surface for the gate metal deposition is fluctuated at a pre-treatment stage, and the evaporation or sputtering process for the gate metal deposition is unstable. Still further, the control for precisely determining the side-etch amount of the insulation film to provide a predetermined size of the second recess is also difficult to be carried out. For these reasons, a GaAs MES-FET having a high performance is not fabricated with high yield.